In or around 1960 a change in the tone of audio amplifiers was apparent. The first transistor radios were being mass produced during this period, and a new process for manufacturing small physically sized signal capacitors was developed and used. The new capacitors were smaller and easier to mass produce.
Most musical instrument amplifiers were still tube amplifiers at this time, but they began to use the modern signal capacitors. This did not affect the tone of musical instruments in the treble register as much as it did musical instruments in the bass register (eg. Tone controls on bass guitars no longer changed the tone very noticeably, compared to what they would do when playing through an amplifier made before the advent of the new process small physically sized signal capacitors.)
In the late 1960's through the 1980's most musical instrument amplifiers began to be produced as solid state amplifiers. These amplifiers were not an improvement in tone, but an improvement in manufacturing process. By the 1980's certain amplifier and effects manufacturers began to introduce 12AX7 tube preamplifier circuits that were fairly effective in making the tone of treble register instruments richer and less distorted. Bass register instruments were not affected to the same extent, and new ways of playing the electric bass (e.g. slap picking, etc.) were developed.